It became a metaphor back around when they released Empire, Total War, (edit: 2012 ish maybe,) and the disk meant nothing and just sent you to Steam to download it, forcing you to keep steam on your computer that tried to run at all times, and make you ask for permission every time you wanted to play the game, revocable at any moment I presume.

It's a metaphor in the sense that if they didn't get rid of physical disks, they would still be putting games on them that don't work without an internet connection and cannot be resold.